1. Field of the Invention
The present invention generally relates to communications systems for use in oilfield applications and more particularly to an apparatus and method for transmitting acoustic signals between a surface location and a downhole location in a well.
2. Description of the Related Art
The control of oil and gas production wells constitutes an on-going concern of the petroleum industry due, in part, to the enormous monetary expense involved as well as the risks associated with environmental and safety issues.
One type of conventional production system utilizes electrical submersible pumps (ESP) for pumping fluids from downhole. In addition, there are two other general types of productions systems for oil and gas wells, namely plunger lift and gas lift. Plunger lift production systems include the use of a small cylindrical plunger which travels through tubing extending from a location adjacent the producing formation down in the borehole to surface equipment located at the open end of the borehole. In general, fluids that collect in the borehole and inhibit the flow of fluids out of the formation and into the well borehole are collected in the tubing. Periodically, the end of the tubing is opened at the surface and the accumulated reservoir pressure is sufficient to force the plunger up the tubing. The plunger carries with it to the surface a load of accumulated fluids that are ejected out the top of the well thereby allowing gas to flow more freely from the formation into the borehole to be delivered to a distribution system at the surface. After the flow of gas has again become restricted due to the further accumulation of fluids downhole, a valve in the tubing at the surface of the well is closed so that the plunger then falls back down the tubing and is ready to lift another load of fluids to the surface upon the reopening of the valve.
A gas lift production system includes a valve system for controlling the injection of pressurized gas from a source external to the well, such as another gas well or a compressor, into the borehole. The increased pressure from the injected gas forces accumulated formation fluids up a central tubing extending along the borehole to remove the fluids and restore the free flow of gas and/or oil from the formation into the well. In wells where liquid fall back is a problem during gas lift, plunger lift may be combined with gas lift to improve efficiency.
In both plunger lift and gas lift production systems, there is a requirement for the periodic operation of a motor valve at the surface of the wellhead to control either the flow of fluids from the well or the flow of injection gas into the well to assist in the production of gas and liquids from the well. These motor valves are conventionally controlled by timing mechanisms and are programmed in accordance with principles of reservoir engineering which determine the length of time that a well should be either xe2x80x9cshut inxe2x80x9d and restricted from the flowing of gas or liquids to the surface and the time the well should be xe2x80x9copenedxe2x80x9d to freely produce. Generally, the criterion used for operation of the motor valve is strictly one of the elapse of a preselected time period. In most cases, measured well parameters, such as pressure, temperature, etc. are used only to override the timing cycle in special conditions.
It will be appreciated that relatively simple, timed intermittent operation of motor valves and the like is often not adequate to control either outflow from the well or gas injection to the well so as to enhance well production. As a consequence, sophisticated computerized controllers have been positioned at the surface of production wells for control of downhole devices such as the motor valves.
In addition, such computerized controllers have been used to control other downhole devices such as hydro-mechanical safety valves. These typically microprocessor based controllers are also used for zone control within a well and, for example, can be used to actuate sliding sleeves or packers by the transmission of a surface command to downhole microprocessor controllers and/or electromechanical control devices.
In recent years, production well control systems have evolved to include complex communication requirements for controlling downhole tools such as various pumps and valves. Many control systems utilize information gathered by downhole sensors and transmitted uphole for determining proper valve and pump control settings. The control settings are transmitted then downhole to control the downhole devices.
Telemetry between the surface controllers and downhole sensors and devices is accomplished using a two-way telemetry system. A two-way system is generally required so that information from the sensors such as pressure, temperature and flow can be sent to the surface for use by the controllers. This data is then processed at the surface by the computerized control system. Electrically submersible pumps use pressure and temperature readings received at the surface from downhole sensors to change the speed of the pump in the borehole.
A signal transmitted to the surface from deep within the well requires sufficient power to ensure a signal-to-noise (S/N) ratio capable or providing useful decoding at the surface. The conventional two-way telemetry system suffers in that sufficient power supplies generally require a relatively large volume. Thus requiring complex and/or expensive downhole power supply designs. Therefore a need exists for a two-way telemetry system that provides good S/N ratio and relatively low downhole power requirements.
The present invention addresses one or more of the above-identified problems found in conventional well communications systems by providing a semi-passive two way communications apparatus and method sending an acoustic signal using controlled reflected acoustic energy.
One aspect of the invention is an apparatus for transmitting an acoustic signal between a well borehole first location and a second location comprising a signal generator located at the first location for generating an acoustic source signal. A transmitting medium is operatively associated with the signal generator for carrying the acoustic source signal to the second location. A controllable signal reflector disposed at the second location is used to reflect at least a portion of the source signal, the reflected signal being indicative of a parameter of interest. And a receiver is disposed at the first location for receiving the reflected signal.
The transmitting medium may be fluid in a pipe, fluid between the pipe and borehole wall, the pipe itself or even the earth. A signal generator and receiver are selected according to the desired transmitting medium.
The signal generator might be a fluid pump adapted to transmit acoustic energy into the fluid, or the generator might be a device for transmitting acoustic energy into the pipe or the earth.
The receiver might include a hydrophone, a geophone or an accelerometer depending upon the transmitting medium selected.
The reflected signal may be a bi-level echo signal representing a string of binary states or the reflected signal may be a multi-level echo signal.
Another aspect of the present invention is a method for transmitting an acoustic signal between a well borehole first location and a second location comprising generating a source signal from the first location using signal generator. The method includes carrying the source signal to the second location along a transmitting medium operatively associated with the signal generator and reflecting at least a portion of the source signal with a controllable signal reflector disposed at the second location, the reflected signal being indicative of a parameter of interest. The method also includes detecting the reflected signal at the first location with a receiver disposed at the first location for receiving the reflected signal.